The final blow to kill each of these bosses must be done with any action skill. Equip an action skill in the menu before loading into the mission. Then reduce the boss's health down as low as possible. This is necessary because most action skills are very weak and do not do much damage. Then activate your action skill by pressing the circular button near the middle, left edge of the screen. The circle contains your willpower and show a number representing how many times you can use your action skill. This achievement may be easier to earn in Story Mode (Missions 1 and 2) because you will have more time to build up willpower for using your action skill. If you load up a Mission Mode boss level, you will get only one use of your action skill. As long as your are careful and wear their health down extremely low, it shouldn't really matter. Immediately after killing the second of these bosses with an action skill, the achievement will unlock.